1. Field of the Invention
The invention concerns heat-shrinkable plastic films and more specifically is concerned with disposable diapers that employ such films for such purposes as elastic waist bands.
2. Description of the Related Art
Disposable diapers typically include a liquid-permeable skin-contacting layer, a liquid-impermeable outer shell, and an intermediate absorbent element. In the manufacture of some disposable diapers, strips of heat-shrinkable plastic film are adhesively bonded to the outer shell and then heated to cause them to contract to form shirred, elastic waistband and leg openings. Typically, the outer shell is a thin film of polyethylene which would become distorted if exposed to high temperatures, but at temperatures low enough not to distort the polyethylene film, known heat-shrinkable plastic films contract slowly and hence can require diaper-manufacturing lines to be operated at slower speeds than otherwise would be feasible.
U.S. Pat. No. 4,563,185 (Reiter) describes at length known techniques for applying strips that are contractible, e.g., by heat, to impart both shirring and elasticization at the waistband and/or leg openings of a disposable diaper. The strip employed by Reiter at the waistband is "ethylene propylene rubber blended with ethylene vinyl acetate" (col. 14, lines 30-32) which Reiter says (col. 7, lines 53-62) is described in U.S. Pat. No. 4,303,571 (Jansen et al.). The Jansen patent says that uniaxial orientation of a film of this composition renders it heat-shrinkable, and that exposure of the oriented film to heat results in a return to the unoriented state and the original elastic properties.
U.S. Pat. No. 4,337,771 (Pieniak et al.) describes a film, strips of which are useful for providing shirred waistbands and leg openings. "The strip is intermittently secured to the backing and/or facing in the central portion of the diaper margin to maintain its elasticity; however, the end portions of the strip are continuously . . . secured at the corner portions of the diaper whereby the previously elastic end portions of the strip are effectively rendered inelastic, and the strip reinforces the diaper in such regions" (col. 2, lines 30-39). The strip, usually called an "elastic film member," preferably is made from compositions of A-B-A or A-B block copolymers wherein the A-blocks can be derived from styrene and the B-blocks are derived from conjugated dienes or lower alkenes, which compositions include low-molecular-weight modifiers such as amorphous polypropylene. The elastic film member can be made from a material that becomes elastic when heat shrunk such as a polyolefin or a copolymer of ethylene and vinyl acetate. Upon being secured to the facing and/or backing of the diaper by ultrasonic welding, the elastic film member "is heated at its end portions to a point where it almost melts and loses substantially all of its memory and is thus non-elastic in those areas" (col. 11, lines 2-12).
U.S Pat. No. 4,515,595 (Kievit et al.) describes a disposable diaper with a waistband that can be made elastic by affixing an elastic member or tape made from natural rubber. The elastic member can be stretched to an elongated orientation, affixed to the diaper, and then contracted (e.g., by heating) to gather the diaper material adjacent the elastic member.
U.S. Pat. No. 4,476,180 (Wnuk) describes elastic films made from blends of an elastomeric linear triblock copolymer and ethylene/vinyl acetate (EVA) copolymer. Preferably the end blocks of the elastomeric copolymer are polystyrene, and preferred interior blocks are polybutadiene and polyisoprene. The melt index of the elastomeric block copolymer of Wnuk's elastic films is less than about 10 (Cond. E of ASTM Method D 1238) and that of the EVA copolymer is less than about 0.6 (Cond. B). These elastic films are made from about 40% to about 80% of the elastomeric block copolymer and about 20% to 60% of the EVA copolymer.
Wnuk's elastic films "may be used in a wide variety of applications where a thin, elastic material would be useful. Such films are particularly useful as low cost elastic members for disposable wearing apparel such as diapers and incontinent briefs; they may be used as leg bands or waist bands" (col. 6, lines 41-45). An elastic film of the Wnuk patent has low permanent set in a test wherein the film is stretched to twice its original length, held at that extension for 10 minutes at room temperature and then allowed to relax. In that test, films of Wnuk's examples have a permanent set of from 12 to 24%.
The Wnuk patent also teaches that the elastic films can be made nonblocking "by loading the film surface with small particles or powders such as chalk, clay, silica, and similar materials" in amounts from about 0.5 g/m.sup.2 to about 5 g/m.sup.2 (col. 4, lines 18-29).
U.S. Pat. No. 3,562,356 (Nyberg et al.) describes elastic films made from blends, among which are an elastomeric linear triblock copolymer such as a polystyrene-polybutadiene-polystyrene copolymer and an EVA copolymer. The Nyberg patent says: "Other benefits of the blends as compared with the unmodified block copolymers include the improved resistance of molded articles to distortion and shrinkage at elevated temperatures" (col. 4, lines 51-55).